Thermal cyclers have long been used to repetitively heat and cool devices over wide temperature ranges to validate device designs. In the past, the design validation process has been the most prevalent bottleneck encountered in the development of new solar cell designs for critical mission applications. The customer demand to acquire desired thermal cycles before an urgent launch deadline requires accelerated cycling rates for a thermal cycling validation.
Various types of thermal cyclers have been used to perform the thermal cycling validation and testing process. One example is a conductive thermal cycler that has been used to perform life tests on GaAs solar cells. Cycling is achieved by cooling a fairly massive aluminum plate with counter-flowing liquid nitrogen and then heating the plate with symmetrically embedded electric rod-heaters. The devices under test are held in contact with the plate so that cycling occurs primarily by conduction. Typical solar cells mounted on lightweight 1/4 inch thick honeycomb panels generally require sixty to ninety minutes to cycle between +80.degree. C. and -80.degree. C. while under 1X10E-7 Torr vacuum. The conductive cycler is well suited for vacuum cycling of cells mounted on heavy 1/8 inch thick solid aluminum panels. However, the conductive thermal cycler has poor cycling rates, because the heat and cool phases work against each other in driving the conductive plate to hot and cold temperatures.
Another type of thermal cycler that has been used is the radiant thermal cycler. In the radiant thermal cycler, quartz-halogen lamp radiation is used in a vacuum with a surrounding cold shroud for heat absorption to cycle the cells, as opposed to the direct conduction method employed by the conductive thermal cycler. A shroud is a cooled copper cylinder surrounding the test device and heating lamps inside the vacuum chamber. Cycle periods of thirty to sixty minutes are attainable on lightweight 1/4 inch thick honeycomb panels. Radiant thermal cyclers are well suited for vacuum cycling thin, lightweight specimens with large surface areas. These radiant cyclers have a faster cycling rate than conductive cyclers because only the heat phase works against the cool phase when the heating lamps overcome the cooling shroud effects. The shroud can only recover during the next cool phase even though the shroud is being filled with liquid nitrogen during the heat phase.
Recently, an improved method was used for optimizing the cooling rate for the radiant thermal cycler with the introduction of a small amount of nitrogen gas inside the vacuum chamber so that the conduction of heat from the solar cell coupon under test to the cold shroud is assisted by the nitrogen gas. This nitrogen-assisted cooling is done without a significant degradation of the radiant cooling contribution. The result is a net increase in the cooling rate. It has been experimentally demonstrated that a significant improvement in the cooling late was achieved by maintaining a forty mTorr nitrogen pressure during the cool phase. This pressure yielded shorter cycle periods of twenty-two to forty-five minutes on lightweight 1/4 inch thick honeycomb panels. Under nitrogen gas cooling, the disadvantages remain that the heat phase still works against the cool phase, and only panels of very low mass can be cycled rapidly.
Usually, a single chamber has to be repetitively heated and cooled requiring excessive energy and cyclic time depending on the amount of mass being thermally driven. The required time-consuming thermal cyclic tests are needed to qualify solar cells and other components particularly for space applications. In order to validate solar cell panel designs in a more timely manner, faster thermal cyclers are desirable. With the above designs, thermal life testing of devices could last for as long as several years for 50,000 cycles. Also, during heating and cooling phases, undesirable thermal gradients may be created across the device under test. A common solution in the thermal cycling industry is to ignore this problem by using only one control thermocouple positioned in the middle of the test device for customer data logging. These and other disadvantages are solved or reduced using the present invention.